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brickyard69
10-29-2006, 06:11 AM
I am considering installing a MSD 6AL on my HEI system and the only HEI wiring available says HEI "without" vacume advance.

The motor is a 383 approximate hp is 475.

It is connected to manifold vacume as that is all I could access on the carb.

Could I do without the vacume advance?

Madspeed
10-29-2006, 10:22 AM
Vac advance hooked to Manifold vac will not help performance.
All manifold vac advance will do is add more advance under cruise to increase your mpg. And as you open the throttle and loose vac to the dist your loosing advance as well.

Get the MSD billet dist and the 6Al without vac advance

MonzaRacer
10-29-2006, 10:57 AM
While I can tell you to but a big dollar unit ,,why.
A properly tuned centrifical advance makes best power, then you add in Vacuum advance for mpg.
I see al these cars with no Vac advance and then they lament about 8 or 10 mpg wneh I ran more uncivilized setups and vac advance and never have a problem.
Here is what you do in my experience as a diag tech and race tuner for almost 20 yrs.
Set you initial timing to its best starting position, then you make your base and centrifical your total at the drag strip (you can unhook it after driving if you think it may hurt you) then you play with your advance weights,springs, the slot under the advance plate.
Then you use as much vac advance as you can with out spark knock.
Now this doesnt come into use if the box adds timing in the higher revs.
Too bad Holley dropped the ProStrip Anihilator system as you could taylor the curve pretty good.
If everyone had to learn timing and tuning like idid they wouldnt just try to buy the latest and greatest box, but figure out what actually will work for your combo.
My 78 C10 has a 355, flat top 2 eybrow Federal Mogul Hyprt techs stock bottom end but good rod bolts, and a LT4 hot roller , World SR Torquer heads with 202/160 valves and a tall Holley Street Dominator intak and I adapted my 4011 Holley 800 DP spread boor to it.
I run 16-18 intial all the stock hei centrifical in as fast as stock weights and light springs will give me and manifold vacuum to distributor. This truck is full sized and usually loaded with several hundered pounds of "crap", tools and etc (most likely around and extra 100-1200 pounds of weight) and I drive in soutern Indiana hills and hollers and average 13 -15 mpg with a TH400 and 30x9.50x15 tire out back and what ever gears it has.
Now for fun it has Air Ride Technologies Cool ride with parralel 4 link in back and will cut a corner without good tire/wheel combo yet.
tell me you can get a hoped up truck with no OD trans (in the works) to do the same.
I havent got to cruise it long distance yet as I am still working on fixing the body up and drive it everyday (90 mile round trip ,5 days a week to work).
Now tell me to not run Vac advance and my mile ge goes to about 8 mpg.
Huh uh no way give me advance in my dist, oh and for the reall groaner it ran a points dist with same curve and a MSD 5 hooked up for 8 1/2 yrs before bad wires had me drop in my HEI I had laying around.
Just cuae its out here dont think its better, why not use an HEI and either DUI coil and module or MSD has an HEI module too and with an inexpensive dial back timing light you can make it run awesome.
Oh and if you want a pretty box to stick under hood I got an older GM Bowtie box you can add under hood to fake them out.
Lee Abel
AFTERMARKET PERFORMANCE
PS oh and as for not adding performance then everyone who has EFI cars go pull your est wire and tell us how bad it runs.IF you look at current technology it really speaks for why I use one. Most new cars can advance as muchas 60 degrees and my best distributor I tweaked the crap out of as an experiment wasgiving me 55 degrees total cruising and that 350 would go on to live for almst 8 yrs and 2 cars(timing chain got so bad I repalced it with my current engine.)6 power tours on that engine was pretty cool too and daily driving for work and fun.

Madspeed
10-29-2006, 05:27 PM
Wow that was a story =)

on a side note if you run a lumpy cam your gonna have idle issues with the vac advance

Fuelie Fan
10-30-2006, 07:29 AM
If the car is driven regularly on the street, the economy gain is worth it. I cost myself some business when a guy came into our old shop complaining about mileage and wanted a compete carb tune. I reconnected his vac advance, and never saw him again.

Tom Welch
10-30-2006, 04:58 PM
Why use the MSD box at all? Getting the HEI distributor curved to match the cam profile specs and intended use works great. There is almost nothing better than a good HEI.

brickyard69
10-31-2006, 10:04 PM
I want a rev limiter.

Is there another unit that can be used with a HEI?

Fuelie Fan
11-01-2006, 12:49 PM
http://www.msdignition.com/ignition_13_8364_8225.htm

brickyard69
11-04-2006, 05:27 AM
Fuelie Fan

Thanks. That is perfect!

68Formula
11-09-2006, 11:22 AM
Wow that was a story =)

on a side note if you run a lumpy cam your gonna have idle issues with the vac advance

Odd. I had idle issues with a lumpy cam without it. My engine runs very smooth hooked to manifold vacuum.

As for it losing advance timing due to vacuum, advance will go away as vacuum drops (which makes sense considering it's vacuum advance), but there is a slope (vacuum vs. degrees advance), so it can still help under moderate tip-in conditions.

Keep the advance, just make sure it's adjustable. Tuning the advance (starting, ending, rates, centrifugal, vacuum) is just as important as tuning all the various circuits of your carb.

MonzaRacer
11-10-2006, 02:59 PM
amcmike is right. Vacuum advance will make lumpy cams act smoother . Been there and done that lumpy cam stuff. The whole basis for the term tuning is just that.
If you just buy a box and figure it will make a car run then you need to figure out how a system works.
I used a MSD5 on my 350 roller (LT4 HOT roller, first with Qjet then a 780 Holley and then a 800 Holley) and right now I have 780 back on it but the 800 is back together and soon to go back together.
If you remove the devices that make things work properly then you dont have a good combo.
I tune race cars and guys come to me with timing all crabbed up locked advance plates and stupid stuff on 350-400 hp cars. Now if you have 800 hp race engine then we dont need a lot of timing advance.
Lee

LS6 Tommy
12-24-2006, 08:10 AM
I know this is a lengthy read, but it's good It's a "sticky" at another site I use.

This is a reprint from another board author unknown but wanted to share this with the group as there has been some recent discussions on this.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.


Tommy

CarlC
12-24-2006, 10:55 AM
The above post is spot on. The only thing different that I do is use a Crane adjustable vacuum can. With this I can tune the total amount of vacuum advance and the fully-deployed vacuum setting. Combine this with a properly tuned carburetor and the engine becomes a different animal.

The total amount of deployment can be adjusted on stock-type cans as well by using a homemade slotted tab and a screw to limit the amount the vacuum can shaft moves. To get everything, including the carb tuning right, will take a day or two of trial and error.

brickyard69
12-26-2006, 11:48 AM
Thanks everyone and Happy Holidays!

the camtender
12-26-2006, 06:34 PM
Tommy,thanks for the info.
What or who's vacuum advance dist. would you recommend for a bbc with msd 6al box?
Jerry

MonzaRacer
05-27-2007, 09:49 AM
what ever you want to use just use MSDs wiring diagram all your doing is swapping a box for the module and you will probably have to change coils to match the box.